Circuit breakers

ABSTRACT

A circuit breaker includes an interrupter coaxial with a current sensor or transformer, the combined assembly being encapsulated within solid dielectric material and supported by an earthed tube mounted on an earthed housing. Detection by the sensor of a current overload is communicated over conductors to a circuit within the housing to cause an actuator to pull a dielectric linkage in a direction so as to move an armature through a bell crank and open interrupter, thereby opening the main current path between conductors and flexible coupling. The circuit breaker requires no insulating gas or oil.

This application is a U.S. National Phase Application under 35 U.S.C.§371 of International Application No. PCT/GB01/04103 (published inEnglish) filed Sep. 13, 2001.

FIELD OF THE INVENTION

The present invention is concerned with circuit breakers, particularlyof the type comprising current sensing devices or current transformersto detect primary current overloads or short-circuits for monitoring andprotection.

BACKGROUND

Circuit breakers at medium voltages usually need to employ currentsensing devices or transformers to detect primary current overloads andshort-circuits for monitoring and protection. In indoor applications,these may be incorporated in the equipment of which the circuit breakeris a component part On the other hand, in outdoor applications,particularly in rural electrification schemes, where the circuit breakeris often used in overhead line applications, the current sensor ortransformer is preferably incorporated as an integral part of thecircuit breaker.

This is usually achieved by mounting a current sensor, such as aring-type current sensor or transformer, coaxial with an insulatedconductor or bushing. Typical examples of conventional use are shown inFIGS. 1 and 2. The interrupting device could be typically a vacuum orgas switch.

The design in FIG. 1 usually requires some form of additional liquid orgaseous insulation, such as oil or SF6, to keep the size of the circuitbreaker to acceptable levels and also to ensure that the internalcomponents are maintained free of moisture and contamination. A morerecent design is depicted generally in FIG. 2. Here, the need for a tankfilled with oil or SF6 is removed. The current transformer or sensor ismounted at the side of the switch and electrically in series with it.This example uses a vacuum switch and current transformer encapsulatedin solid insulation.

In both cases, however, it is still necessary for the insulation exposedto outside environmental conditions to have additional “creepage” lengthcompared to insulation that is protected from the external environment.Thus, although the typical design illustrated in FIG. 2 does not needliquid or gaseous insulation material to minimise the overalldimensions, it is still necessary to protect the internal surface 1 ofthe insulation below the switch from the effects of condensation. Inexposed hostile environments, this can only be done in a practicalmanner by filling the volume below the switch with a controlledenvironment such as dry nitrogen or SF6. This requires additional sealsand monitoring and regular maintenance to ensure that the internalsurface does not become contaminated. It is vitally important to ensurethat the internal surface is kept clean and free from condensation andcontamination, otherwise there is a risk of internal electricaldischarge from the live conductor down the insulation to earth.

In FIG. 1, the current flowing through the device is carried byconductors 1, encapsulated in suitable electrically insulating material2, such as epoxy resin or polymer concrete. Connection 9, flexibleconnection 4 and switch 7 provide the internal conducting path.Operation to open or close the switch 7 is performed by actuator 8 andlever 5. The integrity of the internal insulation surfaces is maintainedby using SF6 gas or oil.

In FIG. 2, the current flowing through the device is similarly carriedby conductors 21 and switch 22 through current transformer 23. In orderto maintain the integrity of internal surface 24, some form ofcontrolled environment is required, such as SF6 gas or dry nitrogen.Thus, in both cases it is necessary to protect the internal insulationsurfaces by using a controlled environment, leading to additional costsand also risks of degradation and failure if the controlled environmentis dissipated due to failure of seals and leakage.

SUMMARY OF THE INVENTION

Accordingly, the invention aims to provide a circuit breaker that doesnot suffer from the above disadvantages. To that end, the inventionprovides a circuit breaker comprising an assembly consisting of acircuit interrupter mounted coaxially within a current sensor ortransformer, said assembly being encapsulated within solid dielectricmaterial and supported at one end of an earthed electrically conductivetube whose other end is mounted on an earthed metal housing.

The circuit breaker is preferably a vacuum interrupter.

The circuit breaker is conveniently operated by means of a mechanicallinkage of insulating material extending between said interrupter and anactuator, said linkage being mounted externally of the metal tube andsaid solid dielectric material.

The operating mechanism for the circuit interrupter may be selected fromany of the group consisting of a permanent magnet actuator, aspring-type actuator, a hydraulic actuator, a pneumatic actuator or asolenoid actuator.

The mechanical linkage preferably comprises a rod of solid dielectricmaterial. The ends of the rod preferably pass through flexible bellows,at one end into a space within the dielectric material encapsulating thecircuit interrupter/current sensor or transformer assembly, and at theother end into the said earthed housing.

BRIEF DESCRIPTION OF THE FIGURES

The invention will be described with reference to the followingdrawings, in which:

FIG. 1 is a typical prior art circuit breaker;

FIG. 2 is a later development of a prior art circuit breaker; and

FIG. 3 is an example of a circuit breaker in accordance with the presentinvention.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

In general, a solution to the problems discussed above in connectionwith known circuit breakers is to provide a circuit breaker where thecircuit interrupter 31 is mounted coaxially within the current sensor ortransformer 32. The combination is encapsulated within the mainelectrically insulating body 33, as shown in FIG. 3. By encapsulatingthe interrupter and current sensor or transformer in this way, thesecondary winding can be supported by an earthed metal tube 41. There isthen no internal insulation exposed to high voltage stress, eitherbetween parts at high voltage and earth or across the terminals of thecircuit breaker. This removes completely the need for additionalprotection and regular maintenance.

Referring now in more detail to FIG. 3, an interrupter 31 is connectedbetween conductors 36, 37 constituting the main current path. Coaxiallylocated around the interrupter 32 is a current sensor or transformer 32.The interrupter 31 has an armature 31′ connected via a flexible coupling44 to one of the main current path conductors 36.

Both the interrupter 31 and the current sensor or transformer areencapsulated in a housing 33 of dielectric material. The sensor ortransformer 32 is supported at one end of an electrically conductivetubular body 41, eg of metal, whose other end is electrically andmechanically connected to an electrically conductive housing 44.Secondary wires 43 from the current sensor or transformer 32 can be fedthrough the metal tube 41 to a suitable terminal board (not shown)mounted in the housing 44. The housing 44 and the metal tube 44 areconnected to an earth terminal (not shown).

An actuator 42 is located within the housing 44 and is coupled to alinkage 40, preferably comprising a dielectric rod. One end of the rod40 is coupled to the actuator, for example via a spiral spring, and theother end is coupled to a bell crank mechanism 35. The bell crankmechanism 35 is also coupled to the armature 31′ of the interrupter 31.

When the current sensor or transformer 32 senses an overload current,circuitry on the terminal board senses the overload condition andactivates the actuator 42 in the housing 44 so as to tend to pull thelinkage 40 in direction A. The pull on the link 40 is translated intomovement of the bell crank 35 so as to tend to pull the armature 31′ ofthe interrupter 31 in a rightwards direction (in FIG. 3), whereby toopen the interrupter 31. In this way, the circuit breaker opens the maincurrent path through conductors 36, 37 in response to detection of theoverload current. The interrupter 31 is restored by the action of theactuator 42 pushing the linkage 40 in direction B to close theinterrupter 31 via the bell crank lever 35, whereby to close the circuitbreaker and restore it to its dormant position.

The linkage 40 is preferably terminated mechanically by flexible bellows38 and 39 to provide weather protection where the linkage or drive rod40 enters the housings 33 and 44.

The push-pull motion can be achieved by using a suitable operatingmechanism, such as a permanent magnet actuator as described in UK PatentNo 2297429 or any other form of suitable actuator 42, such as spring,hydraulic, pneumatic or solenoid types.

1. A circuit breaker comprising an assembly consisting of a circuit interrupter between two main current path conductors, the interrupter being encapsulated within solid dielectric material and supported at one end of an earthed electrically conductive elongate support whose other end is mounted on an earthed metal housing, the elongate support also being encapsulated within solid dielectric material, wherein said interrupter is operated by means of a mechanical linkage comprising a rod of solid dielectric material extending between said interrupter and an actuator, said linkage being mounted externally of the conductive elongate support and said encapsulating solid dielectric material, said actuator being selected from the group consisting of a permanent magnet actuator, a spring-type actuator, a hydraulic actuator, a pneumatic actuator, and a solenoid actuator, and wherein the ends of the rod pass through flexible bellows, at one end into a space within the dielectric material encapsulating said interrupter, and at the other end into said earthed housing, whereby no internal insulation is exposed to high voltage stress.
 2. A circuit breaker comprising an assembly consisting of a circuit interrupter mounted coaxially within a current sensor or transformer between two main current path conductors, said assembly being encapsulated within solid dielectric material and supported at one end of an earthed electrically conductive elongate tube whose other end is mounted on an earthed metal housing, the electrically conductive elongate tube suppport also being encapsulated within solid dielectric material, wherein said interrupter is operated by means of a mechanical linkage comprising a rod of solid dielectric material extending between said interrupter and an actuator, said linkage being mounted externally of the electrically conductive elongate tube support and said encapsulating solid dielectric material, and wherein the ends of the rod pass through flexible bellows, at one end into a space within the dielectric material encapsulating said interrupter, and at the other end into said earthed housing, whereby no internal insulation is exposed to high voltage stress.
 3. A circuit breaker as claimed in claim 1, wherein said interrupter is a vacuum interrupter.
 4. A circuit breaker as claimed in claim 2, wherein said actuator is selected from the group consisting of a permanent magnet actuator, a spring-type actuator, a hydraulic actuator, a pneumatic actuator, and a solenoid actuator. 